1. Field of the Invention
The present invention relates to supercharged internal combustion engines in which a supplied air mass flow is measured with the aid of an air mass flow sensor in the air system to determine the air charge of cylinders of the internal combustion engine.
2. Description of Related Art
To detect the air charge of an internal combustion engine, an air mass flow sensor, e.g., in the form of a hot-film air mass meter, which is situated at the intake of the air system, is normally used. The air mass flow aspirated for operating the internal combustion engine is measured with the aid of the air mass flow sensor and adjusted with the aid of a throttle valve in an intake manifold of the air system or of a controllable supercharger device. In the case of a throttle valve provided in the air system, it is assumed that the air mass flowing through the throttle valve in the air system of the internal combustion engine corresponds to the air quantity aspirated on the intake side. In supercharged internal combustion engines the air is aspirated with the aid of a supercharger device, which is designed, for example, in the form of a turbocharger operated by exhaust gas power to provide a boost pressure upstream from the throttle valve.
Accurately knowing the air mass flows in the air system is essential for properly controlling the internal combustion engine. However, at certain operating points, the measurement of the air mass flow by the air mass flow sensor may be subject to interference. Such operating points are, for example, operating points at which a high boost pressure occurs and little air mass is transported by the compressor. Such operating points occur in particular in the case of a rapid load change from a high to a low load, i.e., when the throttle valve is moved rapidly in its closing direction. So-called compressor pumping may then occur where powerful pressure fluctuations of the air pressure are generated downstream from the compressor of the supercharger device. The reasons for compressor pumping include flow separation at the vanes of the supercharger device's compressor. The pressure fluctuations of the boost pressure also have a negative effect on the air mass flow sensor, so that the measured values detected thereby are highly inaccurate in these operating ranges. For this reason, a substitute value for the air mass flow is provided instead.
In a pneumatic ambient air-pulsed valve, i.e., the ambient air-pulsed valve is not electrically controlled, but only as a function of the pressure differential between the intake manifold pressure and the ambient pressure, the air may also flow back via the air mass flow sensor by flowing from the discharge side of the compressor to the intake side. However, due to its design, the air mass flow sensor is not suitable for detecting a reverse air mass flow with sufficient accuracy. Also in this case, it is better to use a substitute value for the air mass flow.
The substitute value for the air mass flow corresponds, for example, to the air mass flow through the throttle valve, which may be calculated with the aid of a conventional model of the throttle valve and the modeled and/or measured pressures upstream and downstream from the throttle valve. During regular operation of the internal combustion engine, i.e., when the above-mentioned operating point does not exist, the measured value of the air mass flow sensor is used and, at the same time, the model of the throttle valve, which is used for ascertaining the substitute value, is adapted in the known manner. If it is now established that the measured value of the air mass flow sensor should no longer be evaluated because of certain operating states, the ascertained substitute value is used.
Alternatively, for example, in air systems without a throttle valve, the substitute value for the air mass flow may be ascertained via a model on the basis of the intake manifold pressure and the rotational speed.
In supercharged internal combustion engines having an electrically controlled ambient air-pulsed valve in an ambient air-pulsed pipe, which connects the intake side to the discharge side of the compressor of the supercharger device, the use of the substitute value may be made to depend on the activation of the ambient air-pulsed valve. This is, however, impossible in the case of an engine system in which the ambient air-pulsed valve is pneumatically controlled, since no corresponding signal is available.
There are also engine systems in which no ambient air-pulsed pipe bridging the compressor is provided. In this case as well, no control signal for an ambient air-pulsed valve is provided, so that other criteria for the use of the substitute value for the air mass flow instead of the measured mass flow must be used.